Copper ore concentration by induced radioactivity



Filed May 28, 1964 arch 1, 1966 A. M. GAUDIN ET'AL.

COPPER ORE CONCENTRATION BY INDUCED RADIOACTIVITY Concenl'rale 2Sheets-Sheet 1 Photoelectric Timer Relay High Vollage Supply CalculatorOverall Activity Sealer Coincidence Oounler INVENTORS ANTOINE M. GAUDINHARALD F. RAMDOHR United States Patent 3,237,765 COPPER ORECONCENTRATION BY INDUCED RADIOACTIVITY Antoine M. Gaudin, Cambridge,Mass., and Harald F.

Ramdohr, Leopoldshafen, Germany, assignors to Copper Range Company, NewYork, N.Y., a corporation of Michigan Filed May 28, 1964, Ser. No.370,978 7 Claims. (Cl. 209-1115) This application is acontinuationin-part of our application Serial No. 234,741, filedNovember 1, 1962, now abandoned.

This invention relates to the sorting of pieces of copperbearing orehaving varying elemental values and more particularly it relates to themethod and means for separating from ore those pieces having at least apredetermined amount of copper by inducing radioactivity in the ore, andutilizing this radioactivity to effect the separating.

In recent years copper ore has become leaner in grade owing partly tothe depletion of relatively rich deposits and partly to the wider use ofmass mining methods. As a result barren material is taken along with thericher materials in the mining operations. Accordingly, any method ofsorting ore particles to separate barren material from the richermaterials to provide a rich grade feed to the mill is desirable.

Several methods have been proposed to utilize radioactive properties intreating other types of ore to effect the desired separation. Thesemethods can be classified into two groups: one utilizes the naturalradioactivity of the ore; and the other uses artificial radioactivity.In the former case, where natural radioactive ores such as uranium oresare involved, the process of sorting the coarse ore pieces generallyconsists of measuring the gamma radioactivity given off by each piece ina given length of time and then determining the weight of this piece toobtain the ratio of radiation to the weight of the piece. Means havebeen proposed to reject the piece when the ratio indicates theconcentration of radio active material is lower than a predeterminedvalue. The artificial radioactivity is usually induced into the orepieces by absorption of suitable radionuclide from solution or byirradiation with neutrons or gamma rays to create radioactivity in situ.The difliculty in both cases is twofold: (1) how to tag the desiredconstituent in an ore with radioactivity without tagging all thematerials; and (2) how to get rid of the radioactivity in theconcentration after it has served its purpose.

It is the primary object of this invention to use artificialradioactivity as a basis effectively to sort copper ore pieces accordingto the concentration of the desired constituent, and to eliminate theradiation hazards from the final product. This elimination can beachieved simply by waiting as shown by the half-life of Cu 64 in TableI. For example, at the end of five days the activity of Cu 64 is reducedto one-thousandths of the initial activity.

Broadly stated, the invention provides a new method of and means forsorting copper-bearing ore pieces capable of emitting annihilationradioactivity when activated by neutrons. Annihilation radioactivity isa form of radiation generated when a positron combines with an electronto form two gamma rays having equal energy, for example of 0.511 mev.,flowing off at the same instant with the velocity of light and inexactly opposite directions. In the practice of this invention weutilize this special radioactive property by first irradiating thecopperbearing ore pieces to induce artificial radiation which results inannihilation radioactivity and then detecting the gamma radiationsemitted from the newly generated cop per radioisotope, using acoincident count technique to measure the gamma rays due to copper,which are oo- 3,237,765 Patented Mar. 1, 1966 existent within a finitetime interval. This measurement represents the annihilation radiation inthe pieces which in turn is a function of the amount of positron emitterin the ore pieces. By relating this measurement to a predeterminedmineral concentration of a desired constituent, the sorting of these orepieces can be effectively conducted by directing the pieces of orehaving a ratio above this predetermined value to one point and thepieces having a ratio below this predetermined value to another point.

An important feature of this process resides in the waiting timefollowing the neutron irradiation of the pieces of copper-bearing ore.The irradiation forms short-lived isotopes of other elements in the oreand it is only after a waiting time of several minutes, say 10 to 30minutes, sufficient to permit the short-lived isotopes to decay, thatthe radiation detections, measurements and determinations are made toeffect sorting of the desired copper-bearing pieces of ore from otherpieces.

A preferred embodiment of the invention is described hereinbelow withreference to the drawings wherein FIG. 1 is a schematic diagram of asorting apparatus constructed according to the teachings of thisinvention;

FIG. 2 is a plan view of the detector shown in FIG. 1; and

FIG. 3 is a diagram showing a correlation between the trajectory of thepieces and the electric processing, with the sorting device of FIG. 1indicating the ratio of time to distance in the apparatus.

The ore to be used in this example is a shale containing about 53% SiO16% A1 0 7% FeO, 4% MgO, 2.5% K 0, 2% Na O as major constituents. Thecopper content varies from a few hundredths of a percent to severalpercent. A calculation carried out from a more detailed chemicalanalysis on an ore of 1% copper shows the amount of radioactivity ofeach element formed in one kilogram of ore after an irradiation time of0.1 sec. at a flux of 5 10 neutrons cmF/sec. The results of thiscalculation are shown in Table I.

This table lists the amount of radioactivity in microcuries at the endof the irradiation for each radio element formed, the half life of thesame and the gamma radiations emitted, expressed as a percentage of thecorresponding amount of activity. Thus, for Si 31 the amount of activityformed is 1.92 ,uC., of which 0.07% only gives a gamma radiation of 1.62m.e.v.

Table I [Calculated radioactivity of one kilogram of a 1% Cu ore sampleexposed to 5 l0 neutrons/cmF/sec. for 0.1 second] Activity IsotopeObtained, Hall-life Gamma Radiation 1. 91 2. h 0.07% 1.62 rnev. 283. 22.30 1.78 mev.

0. 043 45 (1 53.9% 1.1 mev.45.8% 1.29 mev. 0. 092 2.9 y None. 0. 33 9.7min 58.2% 0.84. mev.41.4% 1.02

mev. 0. 39 15.0 100% 2.75 1nev.100% 1.37

mev 0.0006 d None. 0. 027 8.7 in- 10% 4.05 mew-90% 3.1 mev. 0. 18 5.8111' 94.4% 0.32 mev.others. 5. 3 12.5 h 18% 1.55 mev. 1. 4 2.58 h 50%0.85 mew-others. 6. 4 12.8 h 38% 0.511 rnev.; 1% 1.34 mev. 35.0 5.1 m'9.2% 1.04 may.

Several radionuclides are formed, but it can be seen from Table I thatnot all of these isotopes emit gammarays and that others are relativelyshort-lived. Thus, only Fe 59, Na 24, Mn 56 and K 42 will interfere withthe activated copper. The rapid decay of the shortlived isotopes makesit possible to operate the process on the radiation from the copperwhich has an advantageous radiation life.

Two pieces with a slightly different but low copper content will howevernot be markedly different in overall activity from each other. Thus, anexperiment Was made comparing the activity of high grade ore pieces (6%Cu) against the activity of low grade ore pieces (0.1% Cu). It was foundthat the overall count rate on the average of ten pieces differed onlyby a factor of two, while the copper ratio was about 60.

The coincidence counting method uses the special radiation properties ofCu 64. This isotope with a half-life of 12.8 hours decays by severalsimultaneous ways. One of the decays (39%) gives B radiation. With thiswe are not concerned further. 19% gives off a [3+ radiation which oncontacting electrons in the surrounding matter becomes twice 19% or 38%gamma radiation of 0.511 m.e.v.:

One percent of the radiation is involved in an internal electronactivation stage which then decays with a gamma of 1.34 mev. and theremaining 41% decays with no gamma to Ni 64.

The procedure used depends on the two gammas of 0.511 mev. formed asexplained above. The two gammarays are in coincidence, and they travelin opposite directions.

The detector, shown in FIG. 2, consists of two halves of a cylinder ofplastic scintillator material with a hole 11 drilled along the axis.

A plastic scintillator was selected for several reasons:

(1) Detectors can be cast and machined into any shape desired up to thesize of a barrel.

(2) Plastic materials, in this case polyvinyl-toluene dispersed inanother plastic body, are relatively inexpensive.

(3) The emission time of the light quanta created by gamma impacts isvery short, of the order of 10 sec.

Over a coincidence unit the two halves are connected to a sealer. Thissealer receives pulses when tWo gamma rays hit each detector incoincidence in a time interval of about 10 see. This time interval isdetermined by the resolving time of the electronic parts of the system.coincidences can be counted, when; one of three events happen: (1) truegamma coincidences when a pair of Cu 64 gammas gets into thescintillator; (2) a random coincidence from background occurs; and (3) acascade near-coincidence hits both detectors.

Random coincidences occur when two unrelated gamma rays are reachingboth detectors from a source in a time interval of less than 10* sec.Their numberis related to the resolving time of the device and to theactivity of the background source by N Number of counts in second halfof detector =Resolving time of coincidence counter It is not intendedthat cascade near-coincidences be counted as true gamma coincidences.Yet, cascade neareoincidences can be counted, when radionuclides arepresent which in decaying emit two gamma-rays per atom substantiallyinstantaneously but one after the other, and unoriented in space. Co 60is an example of an isotope decaying according to a cascade pattern. Thenumber of coincidences recorded in the sealer depends on the percentagewith which these cascades occur in the decay scheme of a given nuclide,of their energy, and largely upon the solid angle which the detectorforms around the source. The counting system used for coarse ore such aslumps a few inches in diameter tends to maximize output of true Cu 64coincidences and at the same time minimize counting of random andcascade coincidences. The number of random coincidences is small, if afast circuit is used. To test the importance of random coincidences, aspecial experiment was made. This involved the use of a cesium 137source, which produced a background of 7 10 c.p.m. This gave 700coincidences per minute, or approximately 0.1 percent randomcoincidences.

The apparatus shown in FIGS. 1 and 2 was built on the basis of free fallof the ore pieces through it. The activation of the ore is also carriedout to deal with falling ore pieces. As the speed of free fall increasesfrom the point of release with v=gt (v velocity, g=gravitationalconstant, and t=time), pieces have to be evaluated as close to thereleasing point as possible in order to get a maximum counting time, tout of a given length of detector. By the same token a maximum length ofdetector is also desirable. The third condition is maximum efficiency ofcounter to keep activation level and cost down. This can be done byenclosing the path of free fall within a doughnut or tubelike detector.

In our apparatus, the length of the scintillator is 13 inches. Thisdetector stands upright on a table of laboratory table height. Theactual sorting gauge is 15 inches from the ground to provide space fortwo bins 119 and 20 underneath. The over-all travelling length of an orepiece through the unit from a releasing point 2 inches above thedetector (to provide space for a suitable re leasing mechanism) to theexit into one of the bins is 31.5 inches. The electronic part of thesorting device was arranged according to the schematic sketch in FIG. 1.The ore pieces are first irradiated in a conventional neutron irradiator13. The pieces of copper ore 14 may vary in size, say, from one-halfinch in diameter to about six inches in diameter. In a finite intervalof time sufli= cient to permit the decay of the unwanted short-livedisotopes the activated pieces of ore, one-at-adime, then enter by thefeeder 15, and cut the light beam from light source 16 to aphotoelectric cell 16a. This cell starts the coincidence counter, whichcounts the radiation of each falling piece separately.

The positron emitter (Cu 64) in the falling piece gen= erates theannihilation radiation in the form of two gamma rays 21 and 21a whichtravel in radially opposite directions as shown in FIG. 2, and aretrapped by hemicylindrical scintillators. The photomultiplier tubes 12and 12a connected to the scintillators separately measure these twogamma rays and relay the signal to the coincidence counter.

As mentioned before, the over-all activity of the ore is proportional tothe weight of the material with only very small differences for widelydifiierent copper contents. Obtaining an over-all count from an orepiece thus is equivalent to weighing the piece. As illustrated in thedrawing another pair of photomultiplier tubes 17 and 17a connected to anover-all activity sealer are used to measure the over-all radiation;however, it is possible to obtain the over-all count by suitable wiringof the existing tubes. An electronic division of the number of Cu64-eoincidences by the over-all activity count is obtained from thecalculator. This ratio is a function of the amount of copper presentdivided by the weight of the particle. By comparing this ratio with apredetermined value, which is based upon calculations from data on theactivation, the copper content desired, and the counter efficiency, thecalculator having reached this predetermined value will emit a pulsewhich operates a flipping gate 18. By this time the piece of ore hasleft the scintillators and is dropping through the open gate into theconcentrate bin 19. After a pre-set time, a few milliseconds longer thanit takes the first piece to pass the gate, the whole system is reset bya timer, and is ready to take the next piece. A suitable timer ismanufactured by Industrial Timer Corp., Newark, New Jersey, the typebeing TDAF 115 v., 60 cycles, 1000 w. The time correlation betweendropping of the piece and electric operation is given in FIG. 3.

If a second piece, starting the counter and falling through the system,should not reach the pre-set count because of its low copper content, nopulse goes to the gate and the piece drops right through into thetailing bin 20. The unit is then again automatically reset by the timer.

Various modifications of this apparatus can be made according to theteachings of this invention. For pieces having substantially the sameweight, the over-all activity counter and the calculator can beeliminated. The coincidence counter counts the radiation of the fallingparticle until a pre-set number of coincidence is reached. This pre-setnumber is based upon calculations from data on the activation, thecopper content desired and counter efficiency. Having reached thepre-set number a pulse leaves the counter, which operates the flippinggate 18, to eifect the separation.

Since the ratio obtained from the calculator is a function of the copperassay, by removing the tube 22 the apparatus is converted for assayingthe ore concentrations.

Also instead of using the free-fall principle, conveyor belts, forexample, can be used to carry the ore pieces through various Zones. Theflipping gate can be replaced by controlling a blast of air to separatethe ore pieces.

Table II lists nuclides which can be formed by thermal neutronirradiation, and which give off 5+ particles. These isotopes emitpositrons which result from thermal neutron irradiation.

Table 11 Natural abundance of parent isotope Cross-section, as apercentage barns ot the total amount of that element Isotope Half-life(no: oHoozocuk Table II lists also the half-lites and activationcross-sections for the seven positronemitters. Coincidence counting canbe used for detection and analysis of elements containing theseisotopes.

We claim:

1. A method of sorting copper ore pieces according to a predeterminedmineral concentration which comprises irradiating said ore pieces withneutrons resulting in Cu 64 and other isotopes which are short-lived anddecay at a much faster rate than Cu 64, separately detecting gamma raysfrom said pieces in two semi-cylindrical volumes of space surroundingeach one piece, measuring said gamma rays which are coexistent within 1Oseconds of each other, determining the over-all gamma radiation fromsaid pieces, correlating the measurements of said coincident gamma raysto the determination of overall gamma radiations to obtain the ratiothereof, relating said ratio to said predetermined mineralconcentration, and directing said pieces to one point when said ratio ofsaid pieces is above said predetermined concentration and to anotherpoint when said ratio is below said concentration.

2. A method of sorting copper-bearing ore pieces according to apredetermined copper concentration, said pieces containing varyingconcentrations of copper, which comprises irradiating said ore withneutrons to produce ore pieces containing Cu 64 and other isotopes whichare short-lived, waiting for a time sufiicient to permit selective decayof short-lived isotopes, separately detecting gamma rays travellingdiametrically opposite direction from said Cu 64 .piece, measuring saidgamma rays which are coexistent within a predetermined time interval ofeach other, individually determining the total radiation of said ore,correlating the measurement of said gamma rays to the determination ofover-all radiations to obtain the ratio thereof, relating said ratio tosaid predetermined copper concentration, and directing said pieces toone point when said ratio is above said predetermined concentration andto another point when said ratio is below said concentration.

3. A method of sorting copper-bearing ore pieces according to apredetermined copper concentration, said pieces containing varyingconcentrations of copper and other metals, which comprises irradiatingsaid ore with neutrons to produce ore pieces containing Cu 64 andisotopes of said other metals, waiting for a time sufiicient to permitselective decay of unwanted isotopes, detecting the gamma radiation fromsaid piece of ore, separately detecting gamma rays travelling indiametrically opposite direction from said Cu 64 piece, measuring sai-dgamma rays which are coexistent within a predetermined time interval ofeach other, individually determining the total radiation of said ore,dividing the measurement of said gamma rays in coincidence by the totalradiations to obtain the ratio thereof, relating said ratio to saidpredetermined copper concentration, and directing each piece of oreaccording to the value of said ratio in a direction to effect separationof pieces of ore having a predetermined copper concentration from otherpieces.

4. A method of sorting copper-bearing ore pieces which comprisesirradiating said ore pieces with neutrons forming Cu 64 and short-livedisotopes, waiting for a time suflicient to permit selective decay ofunwanted shortlived isotopes, detecting the total gamma radiation fromeach piece of ore, also separately detecting gamma rays travelling indiametrically opposite directions from pieces of ore containing Cu 64and accepting only gamma rays that are in coincidence, relating theabundance of gamma rays in coincidence to the total gamma radiation ofeach piece to obtain the ratio thereof, and directing each piece of oreaccording to the value of this ratio, in a direction to effect itsseparation from other pieces having copper below the predeterminedlevel.

5. An apparatus for sorting neutron activated pieces capable of emittingannihilation radiation which comprises means to feed said piecesindividually past the center portion of a substantially cylindricalgamma raydetecting means consisting of two hemicylindrical scintillatorsspaced closely apart and connected separately to two photomultipliertubes, coincidence means connected to said two photomultiplier tubes andproviding an output proportional to the number of pulses respectivelysubstantially simultaneously occurring in each of said photomultipliertubes, two additional photomultiplier tubes connected to said twohemicylindrical scintillators, means measuring total outputs from saidtwo additional photomultiplier tubes, correlating means providingoutputs proportional to the ratio of outputs from said coincidence meansand said two additional photomultiplier tubes and flow control meansresponsive to outputs of said correlating means whereby pieces areseparated according to the concentration.

6. An apparatus for assaying neutron activated ore pieces capable ofemitting annihilation radiation which comprises means to move saidpieces individually through a radioactive detecting zone, consisting offirst detecting means capable of measuring the coincidence of gammaradiation and second detecting means capable of measuring total gammaradiation, correlating means receiving outputs from said first detectingmeans and said second detecting means and providing outputs proportionalto the ratio of outputs of first and second detecting means.

7. An apparatus for assaying a particular material capable of emittingpositrons when activated by neutrons References Cited by the ExaminerUNITED STATES PATENTS Pritchett 250-715 X Pritchett 20911l.5Scherbatskoy 250-71.5 X Parker 25083 X M. HENSON WOOD, ]R., PrimaryExaminer.

ROBERT B. REEVES, Examiner.

R. S. SCHACHER, Assistant Examiner.

2. A METHOD OF SORTING COPPER-BEARING ORE PIECES ACCORDING TO APREDETERMNIED COPPER CONCENTRATION, SAID PIECES CONTAINING VARYINGCONCENTRATIONS OF COPPER, WHICH COMPRISES IRRADIATING SAID ORE WITHNEUTRONS TO PRODUCE ORE PIECES CONTAINING CU 64 AND OTHER ISOTOPES WHICHARE SHORT-LIVED, WAITING FOR A TIME SUFFICIENT TO PERMIT SELECTIVE DECAYOF SHORT-LIVED ISOTOPES, SEPARATELY DETECTING GAMMA RAYS TRAVELLING INDIAMETRICALLY OPPOSITE DIRECTION FROM SAID CU 64 PIECE, MEASURING SAIDGAMMA RAYS WHICH ARE COEXISTENT WITHIN A PREDETERMINED TIME INTERVAL OFEACH OTHER, INDIVIDUALLY DETERMINING THE TOTAL RADIATION OF SAID ORE,CORRELATING THE MEASUREMENT OF SAID GAMMA RAYS TO THE DETERMINATION OFOVER-ALL RADIATIONS TO OBTAIN THE RATIO THEREOF, RELATING SAID RATIO TOSAID PREDETERMINED COPPER CONCENTRATION, AND DIRECTING SAID PIECES TOONE POINT WHEN SAID RATIO IS ABOVE SAID PREDETERMINED CONCENTRATION ANDTO ANOTHER POINT WHEN SAID RATIO IS BELOW SAID CONCENTRATION.